A lake near a village suffered heavy mortality of fishes within a few days. Which of the following statements could be the correct explanation for this ?
(i) Lots of urea and phosphate fertillisers were used in the crops in the nearby fields.
(ii) The croplands of the village were fertillised fields rich in nitrate and phosphate.
(iii) The lake water turned green and stinky.
(iv) Phytoplankton populations in the lake declined initially thereby greatly reducing photosynthesis.

Options

A. (i) and (iii)
B. (i) , (ii) and (iii)
C. (ii), (iii) and (iv)
D. (iii) and (iv)

Explanation

To solve the question regarding the heavy mortality of fishes in a lake near a village, we will analyze each of the provided statements step by step.

### Step 1: Analyze Statement (i)
**Statement (i):** Lots of urea and phosphate fertilizers were used in the crops in the nearby fields.

- Fertilizers, particularly urea and phosphates, can run off into nearby water bodies during rainfall.
- This runoff can lead to an increase in nutrient levels in the lake, which may cause excessive growth of algae (algal bloom).

**Hint:** Consider how agricultural practices can impact nearby water bodies through runoff.

### Step 2: Analyze Statement (ii)
**Statement (ii):** The croplands of the village were fertilized fields rich in nitrate and phosphate.

- Similar to the first statement, the presence of nitrates and phosphates in the soil can lead to runoff into the lake.
- This can also contribute to nutrient enrichment in the lake, promoting algal blooms.

**Hint:** Think about the role of different types of fertilizers in nutrient pollution.

### Step 3: Analyze Statement (iii)
**Statement (iii):** The lake water turned green and stinky.

- A green color in the water typically indicates a high concentration of algae due to algal blooms.
- A foul smell can result from the decomposition of dead algae and other organic matter, which can occur when algae die off in large quantities.

**Hint:** Relate the visual and olfactory changes in the lake to the ecological processes occurring.

### Step 4: Analyze Statement (iv)
**Statement (iv):** Phytoplankton populations in the lake declined initially thereby greatly reducing photosynthesis.

- A decline in phytoplankton can lead to reduced oxygen production through photosynthesis.
- However, this statement contradicts the earlier statements that suggest an increase in phytoplankton due to nutrient enrichment.

**Hint:** Consider how the dynamics of phytoplankton populations relate to nutrient levels and oxygen production.

### Conclusion
After analyzing all statements, we can conclude that statements (i), (ii), and (iii) provide plausible explanations for the fish mortality due to nutrient enrichment leading to algal blooms and subsequent oxygen depletion. Statement (iv) is less likely to be a correct explanation as it contradicts the effects of nutrient enrichment.

**Final Answer:** The correct explanations for the fish mortality are statements (i), (ii), and (iii).